Patent document 1 describes an NOx selective reduction catalyst (which will be also referred to simply as “NOx catalyst” hereinafter) and a PM sensor provided downstream of a filter for trapping particulate matter (which will be sometimes referred to as “PM” hereinafter) in exhaust gas.
Patent document 2 describes that deposition of PM contained in exhaust gas on a catalyst deteriorates the catalytic effect.
Patent document 3 describes that when it is determined that a catalyst is in a poisoned condition, a determination as to the degree of deterioration of the catalyst is disabled, or outputting of a result of a determination as to the degree of deterioration of the catalyst is disabled.
Patent document 4 describes that adhesion of HC, SOF, and PM etc. to an exhaust gas purification catalyst leads to a deterioration (poisoning) of the purifying function.
Patent document 5 describes that since liquids such as condensed water have electrical conductivities higher than PM, whose main ingredient is carbon, a change in the capacitance caused by adhesion of condensed water to an electrode part of an PM sensor is significantly larger than a change in the capacity caused by adhesion of PM to the electrode part of the PM sensor.
Even if a filter is provided upstream of an NOx catalyst, a certain amount of PM passes through the filter, and consequently PM adheres to the NOx catalyst in some cases. For example, if the filter has a crack, a large amount of PM adheres to the NOx catalyst. If the surface of the NOx catalyst is covered with PM adhering to the NOx catalyst, the hydrolysis of urea is hindered. In consequence, urea passes through the NOx catalyst without being used to remove NOx in the NOx catalyst. If such urea adheres to the PM sensor, the urea affects the output value of the PM sensor, possibly leading to a deterioration in the accuracy of PM detection. In the case where a cover for protecting a PM sensor element is provided, if reducing agent adheres to the cover, PM is hard to reach the element, possibly leading to a deterioration in the accuracy of PM detection. Although a failure of the filter can be determined using the PM sensor, adhesion of urea to the PM sensor makes the determination of filter failure difficult.